1. Field of the Invention
The present invention relates to a binder composition for mold making which is useful in making a casting mold, especially a self-hardening mold, a composition for casting mold making comprising the binder composition and a specific curing agent composition, a sand composition for mold making containing the binder composition or binder/curing agent composition, and a process for making a mold using the sand composition.
2. Discussion of the Background
Self-hardening molds have conventionally been produced by mixing a granular refractory material, such as silica sand, with a binder comprising an acid-curable resin and a curing agent, charging the mixture in a pattern, and causing the binder to cure.
In an early stage of the art, a thick solution of phosphoric acid had been widely used as a curing agent for the acid-curable .resin. For example, JP-B-39-1543 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses the use of an acid-curable resin, such as an acid-curable furan resin, as a binder and use of an aqueous solution of an acid, e.g., phosphoric acid, as a curing agent for the binder. JP-A-47-16324 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses the use of phosphorous pentoxide (P.sub.2 O.sub.5) as a curing agent.
However, since the conventional binders generally have a low curing rate, the strength of the mold obtained in the initial stage (initial strength) is low. That is, a long time is required from the filling of the pattern to the removal of the cured mold from the pattern, resulting in poor utilization of the pattern.
In order to overcome this disadvantage, an increase in the amount of curing agent has been attempted. However, if mold making is repeated while using a large quantity of phosphoric acid as a curing agent, phosphorus is gradually accumulated in the reclaimed sand to a large quantity, which often causes casting defects, such as pinholes. Further, since the reclaimed sand absorbs moisture due to the accumulated phosphorus, it tends to provide a casting mold having a deteriorated strength.
For these reasons, an aromatic sulfonic acid, such as benzenesulfonic acid, toluenesulfonic acid or xylenesulfonic acid, which has high reactivity to increase the curing rate of an acid-curable resin has been used as a curing agent in place of phosphoric acid. However, aromatic sulfonic acids decompose due to the heat of a casting material, whereby it releases poisonous decomposition products, such as sulfur-containing gas. Therefore, the use of an aromatic sulfonic acid as a curing agent is detrimental to the working environment.
Hence, alteration of the kind of a binder itself has been attempted instead of increasing the amount of a curing agent or changing of the curing agent. For example, JP-A-56-56753 proposes to incorporate, into furfuryl alcohol, a resol type phenol-formaldehyde resin substantially free from a mononuclear phenolic compound, a binuclear phenolic compound or a methylol derivative thereof. The use of such a resol type phenol-formaldehyde resin substantially free from low-molecular weight resins as part of a binder brings about reduction in time required for high-molecular forming reaction, thereby improving mold strength in the initial stage.
However, there remains a need for binder compositions for mold making which provide a mold with high initial strength and do not suffer from the above-described drawbacks.